#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#define deb(...) logger(#__VA_ARGS__, __VA_ARGS__)
template<typename ...Args>
void logger(string vars, Args&&... values) {
cerr<<vars<<" = ";
string delim="";
(...,(cerr<<delim<<values,delim=", "));
cerr<<"\n";
}
#else
#define deb(...) logger(#__VA_ARGS__, __VA_ARGS__)
template<typename ...Args>
void logger(string vars, Args&&... values) {}
#endif
#define pb push_back
#define sz(x) (int)(x.size())
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
template<typename T> inline void maxa(T& a,T b){a=max(a,b);}
template<typename T> inline void mina(T& a,T b){a=min(a,b);}
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
const int mxN=1e5+5; //make sure this is right
const int mod=1e9+7;
int dp[2][4][4][4][4];
int id[1000];
int calc(int a,int b,int c){
set<int> res={a,b,c};
int k=*res.begin();
if(!k)
res.erase(k);
return sz(res);
}
int main(){
cin.sync_with_stdio(0);
cin.tie(0); cout.tie(0);
#ifdef LOCAL
freopen("input.txt","r",stdin);
freopen("output.txt","w",stdout);
#endif
int n; cin>>n;
string s; cin>>s;
id['M']=1;
id['F']=2;
id['B']=3;
memset(dp,-1,sizeof(dp));
dp[1][0][0][0][0]=0;
int ans=0;
for(int i=0;i<n;i++){
memset(dp[i&1],-1,sizeof(dp[i&1]));
for(int a=0;a<4;a++){
for(int b=0;b<4;b++){
for(int c=0;c<4;c++){
for(int d=0;d<4;d++){
if(dp[i&1^1][a][b][c][d]==-1)
continue;
int k=id[s[i]];
maxa(dp[i&1][b][k][c][d],dp[i&1^1][a][b][c][d]+calc(a,b,k));
maxa(dp[i&1][a][b][d][k],dp[i&1^1][a][b][c][d]+calc(c,d,k));
maxa(ans,dp[i&1][b][k][c][d]);
maxa(ans,dp[i&1][a][b][d][k]);
}
}
}
}
}
cout<<ans<<"\n";
}
